1. Field of the Invention
The present invention relates to optical transmission systems, and more particularly, to an optical transmission system for switching optical packets for transmission.
2. Description of the Related Art
In recent years, technology called optical interconnect has been researched and developed. Optical interconnect is a generic term referring to optical data communications over very short distances and usually signifies optical communications over shorter distances than LANs.
Optical interconnect is roughly classified into three types of optical interconnection, namely, optical interconnection between devices (such as communication between personal computers), optical interconnection between boards (communication between printed boards), and optical interconnection within a board (communication within a printed board). Conventional metal interconnection is associated with problems such as transmission loss and constraints on transmission bandwidth, but by using optical fibers, attenuation of signal strength and transmission bandwidth can be remarkably improved.
Meanwhile, the performance of CPUs has been noticeably advancing in recent years. There is, however, a considerable gap between the rate of advancement of LSI chips such as CPUs and that of peripheral technology associated with electrical wiring on printed boards. Also, with the rapid, unceasing improvement in the performance of LSI chips, the number of input/output pins necessary for exchanging signals has become as large as several thousands.
With techniques deriving from the existing electrical wiring technology, it is impossible to cope with such an enormous number of pins. For this reason, in-board optical interconnect has been attracting attention as a breakthrough in solving the problem of wiring bottleneck.
Because of the wide transmission band characteristic, application of optical interconnect to other fields is also pursued, such as signal switching in a parallel computer system including supercomputers connected to one another or in high-speed routers, in order to avoid the bottleneck (bandwidth or resources) of the electrical wiring technology.
Many of optical interconnect systems introduced until now adopt switching techniques in which optical signals are once converted to electrical signals for switching. With this configuration, however, broadening of the bandwidth entails increase in the number of switching ports. Accordingly, attempts are being made to realize optical packet switches whereby optical signals are directly switched, thereby to reduce the scale of switches.
As conventional optical switch-related techniques, a technique of synchronizing an optical communication network to lessen variations in frame reception timing of nodes has been proposed (e.g., Unexamined Japanese Patent Publication No. H02-186898 (pages 646 to 648, FIG. 5)).
In cases where optical packet switching is carried out on the optical interconnect system, optical packets arrive at the respective input ports of the optical switch at different times.
A buffer device (delay device) capable of retaining an optical packet as it is and compensating for an arbitrary arrival time difference does not exist. Conventionally, therefore, in order to correct arrival time differences of incoming packets, a guard time has been used in conjunction with the optical packet transfer control. However, the guard time is a non-transmission time period carrying no information, and since the guard time is lengthened with increase in the arrival time difference, a problem arises in that the optical packet transmission efficiency noticeably lowers.
FIG. 12 illustrates the problem caused by the arrival time differences of optical packets. An optical transmission system 5 includes transmitters 51 to 53, receivers 54 to 56, and an optical switch 57. Optical packets transmitted from the transmitters 51 to 53 are switched by the optical switch 57 to be sent to the receivers 54 to 56. Each of the optical packets transmitted from the transmitters 51 to 53 has guard times provided at the head and tail thereof.
The optical switch 57 switches, at identical switching timing, optical packets input thereto from its individual ports. In FIG. 12, rectangular wave-like dashed lines illustrate a packet switching process performed at the same switching timing irrespective of input ports, wherein the optical packets are switched at timing t1 and are output at timing t2.
The optical packet p1 transmitted from the transmitter 51 fits in the timing range t1, including the guard time provided at each end thereof. Accordingly, the optical packet p1 is normally switched at the timing t1 and output at the timing t2 to the receiver 54.
On the other hand, the optical packet p2 transmitted from the transmitter 52 arrives at the optical switch 57 after a delay of time d1, compared with the optical packet p1, so that only the former part of the payload of the optical packet p2 fits in the timing range t1, with the latter part of the payload being left behind. Consequently, only the former part of the payload is switched and is output to the receiver 55 at the timing t2.
The optical packet p3 transmitted from the transmitter 53 arrives at the optical switch 57 earlier than the optical packet p1 by time d2, so that only the latter part of the payload of the packet p3 fits in the timing range t1, with the former part of the payload being left out. Thus, only the latter part of the payload is switched and is output to the receiver 56 at the timing t2.
In order for the optical packets p2 and p3 to be switched normally without any part of their payloads being lost, it is necessary that the payloads of the individual optical packets be shortened in advance to secure longer guard time intervals.
Thus, in the conventional optical transmission system 5, where the arrival time differences of optical packets are large relative to the switching timing of the input ports of the optical switch 57, the guard time interval for correcting the arrival time differences needs to be set long, and since a longer guard time interval entails a corresponding decrease in the transmittable amount of data, a problem arises in that the data transfer efficiency lowers.
On receiving optical packets switched in the aforementioned manner, the receivers extract and recover clock signals from the received optical packets. The conventional optical transmission system 5 is also associated with a problem that the clock signals extracted from the individual optical packets involve a bit phase shift.
In the optical transmission system 5, burst optical packets are received and the clock signals and data are recovered from the respective optical packets. Even if the aforementioned arrival time differences of optical packets transmitted from different terminal nodes could be corrected, the bits still remain unadjusted, with the result that electrical signals recovered from the optical packets involve a bit phase shift associated with each packet.
FIG. 13 illustrates the manner of how optical packets are received. In the illustrated optical transmission system 5, optical packets #1 to #3 transmitted from the transmitters 51 to 53, respectively, are switched by the optical switch 57 to be sent to the receiver 54. The receiver 54 receives the switched optical packets #1, #2 and #3 in this order.
FIG. 14 shows bit phase shifts. The receiver 54 extracts a clock signal ck1 from the received optical packet #1, then extracts a clock signal ck2 from the received optical packet #2, and extracts a clock signal ck3 from the received optical packet #3.
The clock signals ck1 to ck3 extracted from the respective optical packets #1 to #3 should originally be synchronous (where the three optical packets #1 to #3 altogether represent a single piece of information, for example, the clock signals extracted from the optical packets #1 to #3 must be synchronous). However, because of differences in environmental condition between the optical fiber transmission paths from the transmitters 51 to 53 to the receiver 54, for example, the optical packets transmitted over the optical fiber transmission paths via the optical switch 57 develop bit-level phase shifts, causing synchronization error of the clock signals ck1 to ck3 extracted from the optical packets.
Thus, in cases where burst optical packets are received and clock signals are extracted from the respective optical packets, the extracted clock signals involve bit phase shifts, giving rise to a problem that data cannot be recovered with high accuracy.